A process of liquefying, for example, natural gas (NG) as liquefied natural gas (LNG) employs a so-called “closed loop type” in which a refrigerant having a specific composition (for example, nitrogen (N2) and a mixed refrigerant) is used and the refrigerant for exclusive use is circulated as a closed system. Therefore, there are following issues as the small- and mid-sized liquefaction process of natural gas in which a simple apparatus is desired.
1) A refrigerant manufacturing facility and a storage facility are required, or when the refrigerant is not manufactured, it is required to purchase the refrigerant.
2) When a mixed refrigerant is used as the refrigerant in the closed loop type, if a feed composition changes, the refrigerant composition needs to be adjusted, which is troublesome. Further, because mixing of the refrigerants needs to be performed accurately, time is required for the startup and the plant stability. Therefore, if shut-down and restart are repeated frequently, this process is not suitable.
3) When nitrogen (N2) is used as the refrigerant in the closed loop type, it is generally required to boost the nitrogen refrigerant pressure to a high pressure equal to or higher than 80 kg/cm2. Therefore, facilities such as a compressor and supply facilities such as piping and valves become expensive.
Therefore, in recent years, a technique of an open loop cycle process in which the natural gas is directly used as the refrigerant has been proposed (Patent Literature 1).